Modern motor vehicles include a multitude of electronic control units, which are installed at different locations in order to assume control or regulation functions. Control units are also used as embedded systems for the control and regulation of machinery, plants and other technical processes. In order to allow a safe operation of a control unit in installation locations where discharges of static electricity (ESD) and other potentially interfering electromagnetic effects are to be expected, adequate electromagnetic compatibility (EMC) of the control unit must be ensured.
Industry standards such as ISO 10605:2008 prescribe specific test methods by which the electromagnetic compatibility of a control unit must be checked, in particular with regard to electrostatic discharges. For this purpose, the running control unit is selectively exposed to the effects of an air discharge in close proximity to its housing, and it is monitored whether malfunctions arise in the electronics of the control unit. The mechanism that leads to the malfunction in the electronics system is that the high electrostatic field affects the voltage level of high Ohmic circuit components (e.g., oscillator circuits, inputs of analog-digital converters and open-circuited pins on ICs with weak pullup sources such as test pins, programming pins, sensors). If the voltage level of a circuit experiencing such interference is outside its function range, the electronic system may malfunction.
FIG. 5 shows a cross-sectional view of a conventional control unit 100 for a motor vehicle, which is mounted on a test device 500, while a test for electromagnetic interference is implemented during electrostatic discharges according to ISO 10605:2008, the test being shown in simplified form. Control unit 100 has a housing 101 made of a non-conducting material, and a circuit board 102, which is fixed in place therein and carries electronic components, of which only an integrated circuit 112 having an open-circuited input 104 is shown here for reasons of clarity.
To perform the test, control unit 100 was placed on a horizontal metal plate 154 of testing device 500, the metal plate having an electrical connection to ground potential 156. Via a cable tree 158, the control unit is electrically connected to a load simulator 160, which simulates a load to be controlled by control unit 100 during an actual operation and includes an electrical connection 161 of an internal ground potential of control unit 100 to grounded metal plate 154. An ESD gun 150, which is part of test device 500 and used to generate electrostatic discharges at a tip 151 of gun 150, is likewise connected to grounded metal plate 154, via a flexible line 152. As illustrated, tip 151 of the ESD gun is placed on housing 101 of control unit 100 while control unit 100 is running, and an air discharge is applied using a high electrical voltage in relation to zero potential 156, whereupon a check takes place to determine whether a malfunction of control unit 100 occurs. The air discharge briefly causes the formation of an electric field 162, which extends from tip 151 of the ESD gun, through the wall of housing 101, as far as circuit board 102 near open-circuited input 104 of integrated circuit 112. Electric field 162 affects the voltage level at open-circuited input 104, so that a malfunction of control unit 100 occurs.
One strategy for improving the construction of a control unit if such a test method detects malfunctions due to electrostatic discharges consists of enlarging the clearance between the housing wall and the affected circuit component. However, this entails considerable outlay since dies and other production tools must be adapted, and it has the disadvantage of resulting in a larger housing volume.
Another approach consists of increasing the volume conductivity of the housing material through the introduction of electrically conductive materials, which, however, requires a new construction of the housing and entails correspondingly higher costs. Applying a conductive layer on the housing interior likewise causes considerable outlay, since it necessitates additional work steps to apply and contact the layer. Even more involved is the installation of conductive shield surfaces or special encapsulations around the affected circuit components in the control unit.
Therefore, it is desirable to improve the compatibility of a control unit with regard to electrostatic discharges at low cost, in particular as far as constructive modifications are concerned.